Instead of conventional static imaging by radiation and diagnosis of the chest portion using film/screen or photostimulable phosphor imaging plates, there are some attempts to use semiconductor image sensors such as FPDs (flat panel detectors) for imaging dynamic images of the chest portion and applying the images to diagnosis. Specifically, by using the quick response of semiconductor image sensors in the process of reading and erasing image data, pulses of radiation are successively delivered from a radiation source in synchronization with read and erase timing of the semiconductor image sensors to perform imaging several times per second, thus imaging the movement of the chest portion. The series of images acquired by the imaging are sequentially displayed, thus allowing physicians to observe a series of movement of the chest portion associated with breathing movements, heartbeats, or the like.
Various techniques are proposed which extract information useful for diagnosis from dynamic images of the chest portion. For example, Patent Document 1 describes a technique to generate difference images representing differences in pixel value between plurality of image frames constituting a dynamic image and calculate maximum values (average values, intermediate values, or the like) of the pixel values from the generated difference images for display thereof.